Mail: fixed clearing s->passwd in auth http requests.

Mail: fixed clearing s->passwd in auth http requests.

Previously, it was not properly cleared retaining length as part of
authenticating with CRAM-MD5 and APOP methods that expect to receive
password in auth response.  This resulted in null pointer dereference
and worker process crash in subsequent auth attempts with CRAM-MD5.

Reported by Arkadi Vainbrand.